Coated proppants are often used in hydraulic well fracturing to increase production rate of the well. Recently, we have discovered that cured, commercially acceptable, coatings can be applied to proppants using the polyurethane reaction products of polyols and isocyanates. The details of these processes are disclosed in our co-pending U.S. patent application Ser. No. 13/099,893 (entitled “Coated and Cured Proppants”); Ser. No. 13/188,530 (entitled “Coated and Cured Proppants”) and Ser. No. 13/224,726 (entitled “Dual Function Proppants”), the disclosures of which are herein incorporated by reference. Such polyurethane-based proppant coatings are economically and environmentally desirable for a number of reasons, all of which suggest that the development and use of such coating would be highly desirable.
There are many devices, techniques and addition schemes that can be used to bring the reactants into contact with the finely divided, proppant substrate solids. One such device is an electrically powered tumble mixer into which are added the solids and the first coating reactant (typically the polyol component) along with any auxiliary components. After the solids are thoroughly mixed, the second reactant (typically the isocyanate) is metered in to control the polymerization rate of the polyurethane coating on the proppant solid core.
This metering process is important because the polymerization process increases the viscosity of the tumbling, coated, proppant solids with an attendant increase in the power demanded by the mixer device. An overly aggressive addition rate of the second reactant or miscalculation in the amount of proppant sought to be coated in a batch can foul the mixer and burn out the mixer motor as the polymerization reaction proceeds with the attendant viscosity increase. Mixer repair or replacement are expensive propositions that can materially affect the production and economic efficiencies of a proppant coating plant. Adding the second component too quickly can also form viscous, resinous masses of coated solids that adhere to the walls and arms of the mixer. Such masses interferes with the efficient operation of the mixer, require more frequent cleaning and can generate oversized solid masses that cannot be used in the final, coated proppant product.
It would be desirable to have a way to mitigate or control the effects on electrically powered equipment due to the formation of oversized, viscous masses and a potentially harmful electrical load demand increase by the mixer that is associated with the polyurethane polymerization reaction on coated proppant solids.
It would also be desirable to have a mitigation technique that can be used in the coating process safely, inexpensively and without environmental consequence to a variety of polyurethane-based coating formulations.